1. Field of the Invention
The present invention relates to a multi-beam scanning device for use in image forming apparatus such as laser printers, digital copiers, facsimile machines etc. In particular, the present invention relates to a multi-beam scanning device having e.g., a laser diode (i.e., LD) array or a LED array as a light source. In addition, the present invention relates to an image forming apparatus using the multi-beam scanning device.
2. Discussion of the Background
In recent years, a need exists for image forming apparatus such as laser printers, facsimile devices and digital copiers, which perform high speed and high density recording. Therefore, multi-beam scanning devices which scan a recording medium (photosensitive recording medium) such as photoreceptors using plural laser beams to form a latent image thereon have been developed.
As the light source for use in such multi-beam scanning devices, a light source having two light emitting points is disclosed in Japanese Laid-Open Patent Publication No. 8-136841. This multi-beam scanning device includes a stand unit in which a laser unit having a laser chip, which serves as laser emitting elements emitting first and second laser beams, is fixed on a rotatable stand. The stand unit is rotatably held by a holder.
The laser unit is fixed on the rotatable stand after the laser unit is moved in the up-and-down and left-and-right directions to adjust the position thereof while the two laser light emitting elements are lighted. The position of the laser unit is adjusted such that the first beam is located on a rotation axis of the rotatable stand.
Next, by rotating the stand unit, i.e., by rotating the second beam around the first beam, the distance in the sub-scanning direction between the first beam and the second beam is controlled so as to be the same as the distance between the first scanning line and the second scanning line on a recording medium (i.e., recording interval). At this point, the line connecting the two light emitting elements is not parallel to the sub-scanning direction because the distance between the two light emitting elements is larger than the recording interval. Namely, the starting point of a first scanning line is different from that of the second scanning line. Therefore, in order to adjust such start-position difference in the main scanning direction between the first beam and the second beam, timing of emitting the first and second laser beams is electrically controlled.
This multi-beam scanning device has constitution such that the first beam is positioned on the center of the rotation axis of the rotatable stand and the second beam is positioned apart from the center of the rotation axis. Therefore the multi-beam scanning device has a drawback in that when the center of the first beam is fit to the optical axis of a collimator lens, the light path of the second beam is relatively apart from the optical axis of the collimator lens compared to that of the first beam.
Therefore, the beam spots formed on the surface of a recording medium are not uniform. Namely, beam spots having a desired diameter cannot be obtained.
Because of these reasons, a need exists for a multi-beam scanning device which can form plural laser beam spots having a uniform diameter on a recording medium at a high density to perform high speed and high density recording.